CallContext.LogicalGetData gets restored even wher

Question

I noticed that CallContext.LogicalSetData/LogicalGetData don't work the way I expected them to do. A value set inside an async method gets restored even when there is no asynchrony or any kind of thread switching, whatsoever.

Here is a simple example:

using System;
using System.Runtime.Remoting.Messaging;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication
{
    class Program
    {
        static async Task<int> TestAsync()
        {
            CallContext.LogicalSetData("valueX", "dataX");
            // commented out on purpose
            // await Task.FromResult(0); 
            Console.WriteLine(CallContext.LogicalGetData("valueX"));
            return 42;
        }

        static void Main(string[] args)
        {
            using(ExecutionContext.SuppressFlow())
            {
                CallContext.LogicalSetData("valueX", "dataXX");
                Console.WriteLine(CallContext.LogicalGetData("valueX"));
                Console.WriteLine(TestAsync().Result);
                Console.WriteLine(CallContext.LogicalGetData("valueX"));
            }
        }
    }
}

It produces this output:

dataXX
dataX
42
dataXX

If I make TestAsync non-async, it works as expected:

static Task<int> TestAsync()
{
    CallContext.LogicalSetData("valueX", "dataX");
    Console.WriteLine(CallContext.LogicalGetData("valueX"));
    return Task.FromResult(42);
}

Output:

dataXX
dataX
42
dataX

I would understand this behavior if I had some real asynchrony inside TestAsync, but that's not the case here. I even use ExecutionContext.SuppressFlow, but that doesn't change anything.

Could someone please explain why it works this way?

Answer 1

"As expected" in this case is different for different people. :)

In the original Async CTP (which did not modify any framework code), there was no support for an "async-local" kind of context at all. MS modified the LocalCallContext in .NET 4.5 to add this support. The old behavior (with a shared logical context) is especially problematic when working with asynchronous concurrency (i.e., Task.WhenAll).

I explain the high-level mechanics of LocalCallContext within async methods on my blog. The key is here:

When an async method starts, it notifies its logical call context to activate copy-on-write behavior.

There's a special copy-on-write flag in the logical call context that's flipped on whenever an async method starts executing. This is done by the async state machine (specifically, in the current implementation, AsyncMethodBuilderCore.Start invokes ExecutionContext.EstablishCopyOnWriteScope). And "flag" is a simplification - there's no actual boolean member or anything; it just modifies the state (ExecutionContextBelongsToCurrentScope and friends) in a way that any future writes will (shallow) copy the logical call context.

That same state machine method (Start) will call ExecutionContextSwitcher.Undo whenever it is done with the synchronous part of the async method. This is what is restoring the former logical context.